1. Field Of the Invention
The present invention relates to a base current-control circuit, of an output transistor. More particularly, this invention relates to a base current-control circuit which changes the base current of the output transistor as a function of the load current of the output transistor in order to maximize power consumption efficiency.
2. Description of the Prior Art
Typical electronic equipment for processing input signals generally has an output transistor for driving an external device. The output transistor is designed to carry large currents and supplies current from its collector to a load. The current supply from the collector is controlled by the base current.
FIG. 1. shows an output terminal of a typical piece of electronic equipment which comprises an output transistor Q.sub.out, a load R.sub.L, and a source of electric power V.sub.cc. When the input signal processed by the electronic equipment triggers a switching transistor Q.sub.SW, the switching transistor is alternately turned off and on. When the switching transistor Q.sub.SW is turned on, the output transistor is turned on. When the switching transistor Q.sub.SW is turned off, the output transistor is turned off. More specifically, when the switching transistor is turned on, a diode D.sub.1 connecting a transistor base with the collector of the Q.sub.SW transistor is also turned on, and a constant-voltage source loads a resistance R.sub.b with a voltage V.sub.ref. As shown in FIG. 1, node A is at a voltage V.sub.A, which is equal to the total of V.sub.ref and a diode voltage V.sub.D1. At the same time, node B is at a voltage V.sub.B, which is equal to node voltage V.sub.A minus the voltage (V.sub.BE, Q1) between the base and emitter of transistor Q.sub.1. Thus, V.sub.B is equal to V.sub.ref +V.sub.D1 -V.sub.BE,Q1, and if V.sub.D1 is equal to V.sub.BE, Q1, V.sub.S can be V.sub.ref.
The collector current of transistor Q.sub.1, which also functions as base current: I.sub.B for output transistor Q.sub.out, is equal to the node voltage V.sub.B divided by the load resistance across resistor R.sub.b (i.e., V.sub.B /R.sub.b). This is the same as V.sub.ref /R.sub.b, and I.sub.B is constant. Therefore, I.sub.B is determined by the resistance R.sub.b and a constant voltage, and is unrelated to the magnitude of load resistance R.sub.L across the output transistor Q.sub.out. Thus, regardless of the load current I.sub.o, an invariable base current I.sub.B is utilized. As a result, excessive electric power is dissipated unnecessarily. However, it can be appreciated that if the base current I.sub.B were controlled as a function of the magnitude of the load current I.sub.o, electric power would be used more efficiently.